Tripod type constant velocity joints are well known in the automobile industry as one type of constant velocity joints used in the drive system of automobiles to transfer a uniform torque and a constant speed, while operating with a wide range of joint angle. For instance, one example of the tripod type constant velocity joint was illustrated in Japanese Patent Application, S62-233522 as shown in FIGS. 1-2. This tripod type constant velocity joint typically includes tripod 15 fixed to an end of the second rotating shaft, which functions as a driven member, and hollow cylindrical housing 13 fixed to an end of the first rotating shaft 12 which functions as a drive member. Grooves 16 are formed at three locations on the inner face of the housing 13 at equal spacing in the circumferential direction and extend in the shaft direction of the housing 13.
The tripod 15 comprises a boss 17 connected to the second rotating shaft 14, and trunnions 18 each having a cylindrical shape and extending radially from three locations at equal spacing around the boss 17. Each trunnion 18 has a roller 19 fixed at a distal end of the trunnion and with needle rollers 20 engaged therein. In this arrangement, each roller 19 can freely rotate about the trunnion 18 while also be displaced in the axial direction of the trunnion 18. The constant velocity movement between the first and second rotating shafts is ensured with the rollers 19 rotatably and displaceably engaging in the grooves 16 disposed along the inner face of the housing 13. In order to facilitate the sliding movement, a pair of side faces 16a are formed in circular recesses on each side of the respective grooves 16, and each roller 19 is supported rotatably and pivotally along the side faces 16a of the grooves.
As the first rotating shaft 12 rotates, its rotational force is transmitted from housing 13, through roller 19, needle rollers 20, trunnions 18, and to the boss 17 of the tripod 15. This makes the boss 17 rotate, and which also causes rotation of the second rotating shaft 14. When the joint angle of the two rotating shafts 12 and 14 is not zero, a central axis of the first rotating shaft 12 is not aligned with that of the second rotating shaft 14, and each of the trunnion 18 displaces relative to the side face 16a of the guide grooves 16 to move around the tripod 15, as shown in FIG. 1 and FIG. 2. As a result, the rollers 19 supported at the ends of the trunnions 18 move along the axial directions of the trunnions 18, respectively, while rolling on the side faces 16a of the guide grooves 16. Such movement ensures that a constant velocity between the first and second rotating shaft is achieved.
When the first and second shafts rotate with a joint angle present, each roller 19 moves with complexity. For example, each roller 19 moves in the axial direction of the housing 13 along each of the side faces 16a of the respective guide grooves 16, while the rollers 19 change in orientation and further displace in the axial direction of the trunnion 18. Such movement of the rollers 19 cannot cause a relative movement between a peripheral outside face of each of the rollers 19 and each of the side faces 16a to be smoothly made. Thus, a relatively large friction occurs between the faces. As a result, this tripod type constant velocity joint produces three-directional axial forces as the shafts rotate. In the application of a tripod joint to the vehicles, it is known that the axial forces may cause a transverse vibration typically referred to as “shudder”, if a large torque is transmitted with a relatively large joint angle present.
In order to restrain such a shudder problem from occurring, FR 275280 discloses a structure as shown in FIG. 3, and U.S. Pat. No. 6,533,668 B2 discloses a structure as shown in FIG. 4. In the structure shown in FIG. 3(a), roller 19a is guided parallel to the housing groove and spherical trunnion 18 can swing and pivot around an inner spherical roller surface of inner roller 19b. In this case, the contact area between the inner spherical surface of the inner roller 19b and the trunnion 18, when receiving torque for a load, is of an elliptical shape as shown in FIG. 3(b). It has a longer contact diameter “a” and a shorter contact diameter “b”, because a radius “r” of a longitudinal cross-sectional shape of the spherical trunnion 18 is smaller than a radius “r3” of the trunnion 18.
The trunnion and roller structure shown in FIG. 4 has a structure similar to that shown in FIG. 3, however, with certain modifications thereof. The trunnion 18′ has an elliptical shape in the cross sectional view taken normal to the trunnion axis, which comprises a shorter diameter “B” in the length not receiving a load, and a longer diameter “A” in the length for receiving a load. This is to make a contact pattern between the inner spherical surface of the inner roller 19b and the trunnion 18′ closer to a circle, when receiving a torque for a load. As a result, a longer contact diameter a′ in FIG. 4 becomes smaller than the longer contact diameter a in FIG. 3 due to the elliptical shape of the trunnion 18′. However, it still has an elliptical contact pattern even though the degree of ellipse becomes less than that shown in FIG. 3 because a curvature of a longitudinal cross-section of the trunnion 18′ formed by radius r2 and R is not equal to a curvature of an axial cross-section of the trunnion 18 formed by an ellipse 18a defined as a longer diameter A and a shorter diameter B.
Moreover, for manufacturing the constant velocity joint of FIG. 4, there are considerable difficulties not only to machine a complex spherical surface defined by a curvature of a longitudinal cross-section of the trunnion 18′ formed by radius r2 and R and the ellipse shape 18′ defined as a longer diameter A and a shorter diameter B, but also to measure the trunnion 18′ having a complex three dimensional profile, in terms of both inspection and quality control. These difficulties cause the contact pattern mentioned above to be inconsistent in terms of quality, which leads to high costs in manufacturing perspective and also to potential quality control issues.
When these conventional joints rotate with a joint angle present upon receiving loads, as shown in FIG. 3(b) and FIG. 4(b), a pivotal movement of counterclockwise direction of trunnion 18 and 18′ causes a pivotal sliding action to take place on the contact ellipse. Then the pivotal sliding action operates as a frictional spinning moment (of a direction indicated by arrows “Ts” in FIG. 3(b)) so as to change a rolling direction of the roller assembly 19, which comprises the inner roller 19b and the outer roller 19a with needle bearings 20 engaged there-between. As a result, direction of the roller assembly 19 changes and it is in contact with inner or outer face of the guide groove 16, and thus, increasing a frictional contact force there-between. Moreover, the roller assembly 19 displaces not to parallel to the guide groove 16. Hence it is difficult for the roller assembly 19 to be smoothly rolled, and causes a significant rolling resistance.
Moreover, in order to provide grease-entry space for better durability and smooth operation, space “s” is provided between the lower end at the inner face of the inner roller 19b and the spherical face of the trunnion 18, as shown in FIG. 3(a). Alternatively, space “s” can also be provided between the upper and lower ends at the inner face of the inner roller 19b and the upper and lower portions of the spherical face of the trunnion 18, as shown in FIG. 4(a). However, sufficient grease cannot permeate into the space of the upper end between the inner roller 19b and the trunnion 18 easily, in the structure shown in FIG. 3 and FIG. 4. In the case shown in FIG. 3, space is not provided to the upper end of the trunnion, even in the axial direction thereof, and space in the circumferential direction is smaller than that in the axial direction, due to an ellipse contact in which a longer contact diameter “a” in the circumferential direction is bigger than a shorter diameter “b” in the axial direction, thereby blocking grease from the contact area by the difference of the longer contact diameter and the shorter diameter, especially in terms of the circumferential direction, in the structures shown both in FIG. 3 and FIG. 4. As such, because the space is too narrow to accommodate sufficient grease in the circumferential and axial direction, it is difficult for the grease to be introduced into the space. This becomes more problematic when the grease is in high density condition, for example, during the initial driving stage of automobile particularly at a cold outside temperature, which causes a significant rolling resistance in the drive system.
A clearance can be provided between the needle roller 20 and the outer roller 19a and the inner roller 19b, all in the radial direction and in the circumferential direction to reduce the rolling resistance. However, the clearance is not sufficient to facilitate the grease to penetrate between the needle roller 20 and the outer roller 19a and the inner roller 19b, because rims 19a1 and 19a2 inwardly protruded at both circumferential ends of the outer roller 19a block the grease from flowing between the needle roller 20, the outer roller 19a and the inner roller 19b. This inadequate greasing condition causes the needle rollers 20 not to be rolled smoothly between the inner roller 19b and the outer roller 19a. 